If higher requirements in terms of operational, fire, and contact safety have to be met when supplying electrical equipment with energy, the network type of an ungrounded power supply system is used, which is also known as an insulated network (French: isolé terre—IT) or IT system. In this kind of power supply system, the active parts are separated from the ground potential, i.e. against ground. The advantage of these networks is that the function of the electrical equipment is not affected in case of a first insulation fault, such as a fault to ground or a fault to frame, because the ideally infinitely large impedance value prevents a closed circuit from forming between the active conductors of the network and ground in this first fault case. In a three-phase IT system, the outer conductors L1, L2, L3 and, if present, the neutral conductor N are referred to as the active conductors. In a single-phase IT system without a center tap, the two outer conductors L1 and L2 are the active conductors; in case of a center tap, the mid-point conductor is an active conductor, too.
The inherent safety of the IT power supply system thus ensures a continuous power supply of the loads fed by the IT power supply system, namely of the equipment connected to the IT power supply system, even if a first insulation fault occurs.
Hence, the resistance of the IT power supply system against ground (insulation resistance; also called insulation fault resistance or fault resistance in the fault case) is continuously monitored because another potential fault on another active conductor (second fault) would cause a fault loop and the resulting fault current, in connection with an overcurrent protection device, would lead to a shut-down of the installation and to a standstill of operation. According to standard IEC 61557-8, the IT power supply system is monitored by means of an insulation monitoring device, which is connected between at least one of the active conductors of the IT power supply system and ground. In addition, the insulation monitoring device usually comprises an insulation-resistance measuring path having a measuring-voltage source, which superimposes a measuring voltage on the power supply system between the active conductor and ground. Based on a rising measuring current, a deterioration of the insulation state of the IT power supply system is registered and reported in time.
Thus, insulation faults are detected sooner by a factor of 1000 to 1,000,000 compared to grounded power supply systems and, additionally, symmetrical faults can be detected. Moreover, it is possible to measure the insulation resistance in shut-down systems, as well, and insulation monitoring can be performed irrespectively of whether the system is an alternating-current, a direct-current, or a mixed system. Despite these advantages, the IT power supply system is still relatively rarely used in comparison to grounded power supply systems. One reason for this is the problem of excess voltage, which is described below and solved by the invention.
In an IT power supply system, there is the danger that when a low-resistance insulation fault occurs on one of the outer conductors, a potential difference to ground potential (excess voltage) occurs, for which the connected equipment (loads) is not designed. For instance, in case of a dead ground fault of an outer conductor in a three-phase power supply system having a neutral conductor and a nominal voltage of 230 V between the outer conductor and the neutral conductor, the outer-conductor voltage against ground rises by the factor of 1.73 to the maximum value of about 400 V on the two fault-free outer conductors.
This excess voltage is problematic if the fault elimination measures (e.g. Y capacitors) at the connected equipment (loads) are not designed for this increased potential difference.
Moreover, there is higher strain on the insulating materials of the lines, which might be damaged permanently.
While it is possible in a power supply system fully planned as an IT network to configure the fault elimination measures against ground in the loads connected to the IT power supply system for the maximum potential difference from the outset, this is disadvantageous in that, on the one hand, it must be ensured for the entire lifespan of the installation that no (other) loads are used that are suitable for the nominal network voltage only and not for the increased potential difference. This, in turn, requires a technical understanding of the described problem of excess voltage, which in reality is often unavailable on site. On the other hand, grounded power supply systems could be converted into ungrounded power supply systems only by replacing all loads unsuitable for the increased potential difference, which is unpractical for economic reasons.